A seat belt retractor of the above-referenced type with a switchable load limiting device is known, for example from DE 199 27 427 A1. The load limiting device is formed by a torsion rod with two sections having a different diameter which can be selectively switched by a switching device into the force flow between a belt shaft and a blocking device blocking the belt shaft in the direction of extraction. Depending on whether the section of the torsion rod with the larger diameter or the section with the smaller diameter acts between the belt shaft and the blocking device there is then a large belt extraction force limiting or a smaller belt extraction force limiting. The sections of the torsion rod are separated from each other by a multi-tooth profile across which the torsion rod can be coupled with the belt shaft via a torsion bushing. The torsion bushing in turn is coupled to the belt shaft via several catches which are held in the contact position with the torsion bushing by a shaft ring. The switching device is formed by a pyrotechnically driven, displaceable piston, which upon activation of the switching device, displaces the shaft ring, so that the catches move out of contact with the torsion bushing and the force flow between the belt shaft and the torsion casing is interrupted.
Also known from U.S. Pat. No. 6,616,081 B1 is a seat belt retractor of the same type construction in which a shaft ring rests on a rotatable ring with an axially aligned ramp contour, which upon the activation of the switching device is distorted by a piston driven by means of a gas pressure and the shaft ring is axially displaced by sliding off the ramp contour.
The driven piston respectively limits a pressure chamber which upon the activation of the switching device is impacted with a gas flow standing under a high pressure which in turn powers the piston.
A disadvantage of these solutions is that the pressurized gas flow can exit outward when the piston exits the pressure chamber or the pressure chamber is not completely sealed by the piston. Adjacent component parts can be damaged by the exiting gas pressure or can even be set on fire when a pyrotechnical gas generator with a very hot gas flow is used.
The object of the invention is to make available a seat belt retractor with a switchable load limiting device of described above type, in which the gas flow powering the piston is sealed and does not exit the pressure chamber.
The object of the invention is attained by a seat belt retractor with the features described in accordance with this invention. Preferred embodiments are specified in the figures, and the associated description.
The basic idea of the invention proposes that a piston is located on an expandable hollow pot which is attached to a gas generator in a gas-proof manner and forms the pressure chamber. By means of the proposed use of the expandable pot, the pressure chamber impacted with the gas flow is encapsulated in a gas-proof manner with respect to the surroundings, so that it can be prevented with certainty that the pressurized gas flow exits outward. Furthermore, a simple to mount unit is created which includes all component parts required for activation of the switching device. Another advantage of the invention can be seen in the fact that the piston itself no longer must be guided in a gas-proof manner and no pressure chamber has to be created which must be specially sealed in the seat belt retractor. In addition, the construction is simplified as such, since a separate part, namely the piston, can be eliminated, in that the front end of the pot simultaneously performs the function of the piston and conveys the positioning movement to switch the load limiting device.